North America is all mobile party pays, so calls to mobile cost the same as calls to landline.
... not inside the [same provider's] mobile network, cell phone to cell phone. See T-Mobile's "Unlimited Mobile-to-Mobile" component of their services, as an example. This (unlimited, for a flat, usually minuscule, fee) is what I am hoping to achieve with a gateway (making the PBX behind it look like any other mobile phone).
I don't understand where you're planning to save money here. Calls into a mobile network are free for the caller under any circumstances. The free mobile-to-mobile deals apply only to the mobile phone user; if I have it, calls to or from my mobile from other users on the same network are free to me, but the other party still pays whatever his plan says. If your calls aren't placed from a mobile they're free anyway (or whatever your landline rate is, which should be pretty close to free) so where's the savings? If, nonetheless, you want to experiment with this kind of hack, look for devices with names like cellsocket and dock-n-talk that take a cell phone and provide a landline interface that you should be able to connect to a PBX. R's, John